Today's Mobile Backhaul (MBH) networks are usually structured into two parts, the High-RAN (Radio Access Network) and Low-RAN sections in order to provide connectivity and traffic aggregation for mobile packet data from the cell sites to the core network.
As can be seen from FIG. 1 the High-RAN part is typically using a ring topology with the network nodes 110 to 114, while the Low-RAN part is using a tree-structure with the network nodes 121 to 124. The cell sites are connected to the Low-RAN. The Low-RAN is typically using microwave links to the access legs of the High-RAN ring. The head-end of the High-RAN ring connects to the core network using core routers, such as routers 31 and 32 placed on the core site. The edge router connects as well to the core network CN via at least two independent links as shown in FIG. 1.
A ring topology is a standard topology used in current transport network design. In this topology multiple transport nodes are interconnected to each other in the shape of a ring. As show in FIGS. 1 and 2, a ring topology comprises three or more ring nodes 110-114. Each node in the ring has two neighbouring ring nodes and the ring nodes 110-114 may be connected to other nodes in order to support traffic insertion into or extraction from the ring topology.
Ring topologies are used with various transport technologies, by way of example optical transport networks (OTN) or electrical transport networks based on SDH (Synchronous Digital Hierarchy), ATM (Asynchronous Transfer Mode) or Ethernet standards.
Furthermore, a split router architecture is known, a concept which is currently being discussed and under development in various groups, for example in the Forwarding and Control Entity Separation (ForCes) Working Group in IETF (http://datatracker.ietf.org/wg/forces/), the group developing the OpenFlow Protocol, OpenFlow Switch Specification, Version 1.1.0, http://www.openflow.org/ or the recently created Open Network Foundation group, Open Network Foundation website, http://www.opennetworkfoundation.org/.
The split router architecture proposes to split a common router in two elements: a control element (CE) responsible for managing the routing protocol and the connectivity of the data plane. A control element controls the data plane connectivity through a forwarding element (FE). The forwarding element is responsible for forwarding traffic in the data plane and establishes connectivity to a neighbour node based on the instructions received from the control element.
The control element controls a set of at least one FE nodes forming a transport network. FE nodes are connected to each other by transport links between so-called internal ports. FE nodes are connected to nodes outside of the CE-controlled transport network (CE Domain) by so-called external ports. This is shown in further detail in FIG. 3 where a central control entity 200 controls the forwarding elements FE1 to FE5 using a protocol such as OpenFlow. The area controlled by the central control entity 200 is shown by the dashed line 40. Nodes outside this area can utilize the transport services of the CE-controlled network by sending flows to external FE ports, the ports shown by crosses in the embodiment of FIG. 3. The flows contain identifications recognized by the CE 200. The transport service of the CE-controlled transport network is either preconfigured or is configured on the fly by the CE 200 based on the incoming flow identifications. In the embodiment of FIG. 3 the forwarding elements FE1 to FE3 may play the role of the ring network, whereas FE4 and FE5 are forwarding elements, such as forwarding elements 21 and 22 of FIG. 4 on the Low-RAN side.
In FIG. 4 it is shown how the split router architecture on the mobile backhaul is applied. In the embodiment of FIG. 4 the elements containing the nomination FE are forwarding elements such as the forwarding elements 21 to 24 of the Low-RAN and the forwarding elements 10 to 14 in the High-RAN, whereas the other nodes 31 to 35 without the nomination FE are routers. The dashed lines indicate the control plane flow, whereas the solid lines indicate the data plane flow. Between the routers in the CN 31 to 35 control plane flow has not been depicted.
Ethernet is a widely used transport standard that specifies the physical transport layer and part of the data link layer, for example addressing. A ring topology using the Ethernet standard causes some complications. Ethernet and partly protocols above Ethernet provide automatic data path detection and selection. Those protocols must ensure that data are not sent in a loop. Various protocols are proposed to provide loop detection and loop prevention, for example the Spanning Tree Protocol, STP, and improved variants of this protocol like Rapid Spinning Tree Protocol, rSTP. These protocols provide a slow failure detection and failure handling in case a link breaks or a node fails. This failure detection and handling is in the order of seconds, this slow handling is not comparable with fail-over times achieved with the SDH (Synchronous Digital Hierarchy) technology where the failure detection handling is in the order of 50 milliseconds. A new procedure was developed in ITU-T to improve the fail-over time for Ethernet ring topology: the Ethernet ring protection switching, ITU-I G.8032/Y. 1344, Ethernet ring protection switching, http://www.itu.int/rec/T-REC-G.8032-201003-I.
This specification proposes the Ring Automatic Protection Switching (R-APS) protocol to manage the connectivity and node availability in the Ethernet ring. Further functionality defined in the ITU-T recommendation “OAM functions and mechanisms for Ethernet based networks”, ITU-T Y.1731, OAM functions and mechanisms for Ethernet based networks, http://www.itu.int/rec/T-REC-Y.1731-200802-I is used to monitor the availability of links immediately connected to a node.
While OAM (Operations Administration and Maintenance) functions are used in each node to monitor the availability of the directly connected links, the R-APS protocol is used to exchange this information between all nodes in the ring. Finally, each node receives an overview of the availability of links and nodes in the ring. In case of failure, independent decisions are taken in each node to find an alternative route for the traffic bypassing the failed link or node.
The concept of the Ethernet protocols used to prevent Ethernet loops is shown in FIG. 5 and a closed dedicated link for data traffic. This applies for STP, its variants and Ring Automatic Protection Switching (R-APS). In G.8032 the disabled link is called ring protection link, RPL. Ethernet OAM traffic can still pass through the RPL to monitor the link availability but other traffic is prohibited. In the embodiment shown in FIG. 5, the link between ring nodes 110 and 111 is disabled and thus plays the role of the ring protection link in the example shown in FIG. 5.
As a consequence, the traffic cannot take the shortest path in all cases. By way of example, traffic from ring node 110 to ring node 111 has to pass through 114, 113 and 112 as the direct connection to ring node 111 is disabled. The ring protection link impacts the overall transport capacity that can be achieved in the Ethernet ring topology.
Because it is not easy to overcome this situation, the approach in Ethernet based networks is to run each transport node independently from one another. Each node detects the network topology by means of specific topology detection protocols. Based on information gained, each node makes an independent decision based on a common decision model to decide on how to route traffic in a network. The final model shall ensure that the final data paths are always loop-free.
Another drawback is the number of transport labels needed in a MBH to cope also with failover cases. This is a problem for split MBH networks using the split router architecture mentioned above.
A typical deployment scenario is that transport paths (e.g. Multiprotocol Label Switching (MPLS) tunnels, or VLANs) are configured from any access leave of the L-RAN tree to the head-end interface towards the core network as shown in FIG. 6. In order to cater for failure cases in the HRAN ring, the alternative ring label via FE-3 and FE-2 shown in the dashed line needs to be utilized. If e.g. an MPLS mechanism is used, a label swap would be applied at the FE next to the failed link. However, this implies that for each label, which identifies an end point within the MBH, an alternative label needs to be preconfigured. In MBH networks with a plurality of nodes this implies an important administrative effort and the size of the MBH network is limited by the range of possible MPLS labels, divided by two. This implies a hard limitation in the design of MBH. The number of required MPLS labels can be calculated in the following way: the number of labels corresponds to the number of LRAN leaves x 2. As for each LRAN leave towards head-end MPLS tunnel one label for the normal situation and one label for the failure situation is required, the number of labels increases with the number LRAN leaves. If more than one ring is present and traversed in the HRAN, the number of labels doubles with each traversed ring structure. This means that the number of labels corresponds to the number of LRAN leaves x 2 x the number of traversed rings.
The above problem does not only exist in MBH networks but also in other networks such as Metropolitan Area Networks, converged mobile aggregation networks, or Virtual Local Area Networks (VLAN).